1. Field of the Invention
The present invention relates to an oxygen concentration detecting apparatus for detecting an air-fuel ratio, i.e. an oxygen concentration and the like, within the exhaust gas of an internal combustion engine and, more particularly, to an oxygen concentration detecting apparatus for detecting an oxygen concentration utilizing an oxygen sensor of a limiting current type.
2. Description of the Related Art
Hitherto, in an oxygen concentration detecting apparatus of this type, an internal electric resistance of an oxygen sensor of a limiting current type changes with temperature and a current-voltage characteristic that specifies the temperature, i.e. the internal resistance, of the oxygen sensor passes through an origin. The oxygen sensor is positively biased during a first period by a positive voltage and is then negatively biased during a second period by a negative voltage. The currents flowing through the oxygen sensor in the first and second periods are detected to determine an oxygen concentration on the basis of the current detected in the first period and to determine the internal electrical resistance of the oxygen sensor on the basis of the current detected in the second period, as shown in Japanese Patent Laid-Open No. 59-163556 (U.S. Pat. No. 4,543,176), for example. Then, an air-fuel ratio is detected stably on the basis of the detected oxygen concentration after accurately controlling the temperature of the oxygen sensor so as to maintain it in an active state by heating it so that the detected internal resistance becomes almost constant.
Further, the internal resistance and the temperature of the oxygen sensor correspond on a one-to-one basis. Restrictions on the usable temperature range and oxygen concentration measurable range are eliminated by detecting the internal resistance of the oxygen sensor, calculating a voltage to be applied to the oxygen sensor in response to the detected value of the internal resistance and applying the voltage to the oxygen sensor based on that calculation, as disclosed in Japanese Patent Publication Nos. 1-28905 (U.S. Pat. No. 4,626,338) and 1-25419 (U.S. Pat. No. 4,626,338).
In the apparatus constructed as described above, however, there has also been a problem in that because the above-mentioned second period is repeated uniformly, the air-fuel ratio can be determined only after the elapse of the second period and the period in which the air-fuel ratio can be determined is delayed even if the circumstance allows to stably determine the air-fuel ratio. Further, because the above-mentioned second period is repeatedly set regardless of a drop of the temperature of the oxygen sensor, the period in which the negative voltage is applied to the oxygen sensor does not necessarily coincide with the period in which the temperature of the oxygen sensor starts to drop. Accordingly, if the temperature of the oxygen sensor drops too much before the next period in which the negative voltage is applied after a condition in which the air-fuel ratio can be stably determined has been once brought about, it will take a long period of time before the air-fuel ratio can be stably determined, even if the temperature is controlled by applying the negative voltage as described above because the response of the oxygen sensor to the temperature is slow. In such a case, because the above-mentioned second period is set at a time necessary for the internal resistance of the oxygen sensor to be stabilized, the period in which the air-fuel ratio can be determined is delayed even further.
In the apparatus constructed as described above, there has been another problem in that the air-fuel ratio which is detected before the oxygen sensor has been activated is not necessarily credible. In consideration of the operation performance of the internal combustion engine right after its start, it is not necessary to detect an air-fuel ratio, which is likely to be inaccurate after the engine start. Rather, it is more important to raise the sensor temperature quickly to the activating temperature by controlling a heater.